1. Field of the Invention
This invention relates to a method for reducing permeability of a subterranean formation, primarily for use in steam stimulation recovery techniques. More particularly, this invention relates to a method of blocking an area of a subterranean formation by use of a rigid, impermeable foam including alkali metal silicates.
2. Description of the Prior Art
Steam stimulation recovery techniques are widely used to increase production from an oil bearing formation. In steam stimulation techniques, steam is used to heat the section of a formation adjacent to a wellbore so that production rates are increased through lowered oil viscosities and the corresponding reduced resistance to flow through the injected area.
In a typical conventional steam stimulation injection cycle, steam is injected into the desired section of a reservoir. A shut-in or "soak" phase may follow, in which thermal energy diffuses through the formation. A production phase follows in which oil is produced until oil production rates decrease to an uneconomical amount. Subsequent injection cycles are often used to increase recovery.
Steam stimulation techniques recover oil at rates as high as 80-85% of the original oil in place in zones in which the steam contacts the reservoir. However, there are problems in contacting all zones of a reservoir due to heterogeneities in the reservoir such as high/low permeability streaks, which may cause gravity override, and steam fingering. When any of these heterogeneities are present in a reservoir, the efficiency of the process begins to deteriorate due to reduced reservoir pressure, reservoir reheating, longer production cycles, and reduced oil-steam ratios. As a result, steam stimulation may become unprofitable.
Various methods have been proposed so that steam can be diverted to uncontacted zones of a reservoir. One such method is disclosed in U.S. Pat. No. 2,402,588 to Andresen ("Andresen"). Andresen discloses a method of sealing a more permeable area of a reservoir by injecting into a reservoir a dilute alkaline solution of sodium silicate under low pressure. An acid gas such as carbon dioxide is then injected to reduce the alkalinity of the solution, resulting in gelling.
Another such method is disclosed in U.S. Pat. No. 3,645,336 to Young et al. ("Young"). Young discloses the plugging of a zone of a reservoir by injecting a mixture of steam and sodium silicate into the permeable zone. A second mixture containing steam and a gelling agent such as carbon dioxide is injected in the permeable zone, and the two mixtures are allowed to react. A hard silica gel plug is formed.
Another such method is disclosed in U.S. Pat. No. 3,805,893 to Sarem ("Sarem"). Sarem discloses the formation of a gelatinous precipitate by injection of small slugs of a dilute aqueous alkaline metal silicate solution, followed by water and then a dilute aqueous solution of a water soluble material which reacts with the alkali metal silicate to form a precipitate. The precipitate hardens to form a substantially impermeable substance.
Another such method is disclosed in U.S. Pat. No. 3,965,986 to Christopher ("Christopher"). In Christopher, a slug of fumed colloidal silica and water is injected into a reservoir. This slug has a relatively low viscosity. A surfactant is then injected which forms a gel on contact with the silica slug.
In each of the above methods, a relatively impermeable gel can be formed. However, each method requires a large volume of silicate to form a gel with sufficient surface area to ensure that the resulting gel contacts the entire area desired to be blocked.
It is an object of the present invention to provide an improved method of reducing permeability in selected areas of a subterranean formation. It is a further object of the present invention to provide an improved method of reducing permeability in depleted areas of a hydrocarbon bearing reservoir for more effective steam stimulation operations.